1. Field of the Invention
This invention relates to a teletext decoder, and more particularly to a teletext decoder which is of less error even in the receiving condition of being affected largely by noises and can reproduce stable and high quality pictures against variation in environment.
This invention relates further to a teletext decoder having a data slicing circuit which slices information (to be hereinafter called the teletext signal) superposed during a predetermined horizontal scanning period in the vertical blanking period of composite television signals and converts the sliced information into the binary digital signal.
2. Description of the Prior Art
The teletext broadcast has already been carried out in U.K. Also, in Japan, a teletext system using the pattern transmission system was accepted by the Radio-acoustics Inquiry Commission in March, 1981. The picture data of these systems all are binary non-return-to-zero (NRT) signals which are superposed in unit of one horizontal scanning period (1H) during the vertical blanking period of the video signal. FIG. 1 shows waveform charts of the superposed teletext signals. In FIG. 1-(A), reference numeral 1 designates a horizontal synchronizing signal, 2 designates a color burst signal, 3, 4 and 5 designate binary signals to be superposed, a portion 3 showing a clock run-in signal (to be hereinafter called CR), a portion 4 showing a framing code signal (to be hereinafter called FC), a portion 5 including various data information, 6 designates the enlarged CR, and 7 designates the enlarged FC. CR is the synchronizing signal for reproducing the data sampling clock signal and FC is for synchronizing the data packet, the CR and FC being common to all the teletext data. The teletext decoder can receive and reproduce the subsequent data with accuracy by detecting FC. Accordingly, it is very important for the reception performance to enable the data read-in sampling clock signal to be synchronized with CR and also FC to be detected stably at the regular timing. If FC is not detected at the regular timing, or at the abnormal timing, erroneous signals are received to lead to a display of random picture. Since FC is important as described above, the teletext decoder is designed to have an error-correcting function for one bit so that even when a one bit error is generated by noise, FC is adapted to be detectable with accuracy.
FIG. 2 is an illustration of framing code detection, showing each step of sequential arrival of teletext signal from CR and the number of bits coincident with that in the comparison byte. In FIG. 2, the steps (1) through (7) are for comparison of CR, the step (8) shows arrival of the first one bit of FC, and the following steps are shown at every clock signal. The step (15) is the moment when the arrival bits all are coincident with the comparison signals, at which time FC detection pulse is generated. The numbers of coincident bits prior to the step (15) all are 5 or less, so that when the number of coincident bits is 7 or more in the teletext decoder, the FC detection pulse is generated, resulting in that the one bit error correction can be performed.
Now, it is necessary for detecting the FC in the actual teletext decoder to slice the central portion of amplitude of teletext signal superposed on the video signal to thereby obtain the wave-shaped slice data signal and sampling clock signal in phase synchronized with CR. The slicing point in the teletext signal is required to be accurately sliced at the central portion, even when the amplitude or DC level thereof is different between the broadcasting stations or between the performances of teletext decoders. Generally, the slicer circuit is designed to detect its center level by a proper time constant during the CR period and thereafter keep the level in a value within a fixed range. Hence, the slice data signal in the vicinity of CR, especially in the first half, is occasionally different from the transmitted signal. The follow-up property of the above slicer circuit will decide a timing at which the data becomes normal, but this timing should not be too early in order to improve the antinoise characteristic. The sampling clock signal reproduction, as the same as the above, is synchronized in phase with CR to be normal in phase, but a time to draw the clock signal into the normal phase is required and the clock phase in the first half of CR is not normal. Moreover, when in the weak electric field, the jitters of the above slice data signal and sampling clock signal become larger. The FC detection circuit, which has the one bit error correcting function as aforesaid, conversely is liable to detect FC rather by mistake at the timing when the number of coincident bits in FIG. 2 is 5, especially in the first half of CR, there is a high probability of detecting FC by mistake.
Conventionally, a means to solve the above problem is to generate gate pulse (to be hereinafter abbreviated to the FC gate pulse) from the horizontal synchronizing signal, therby allowing only the FC detection pulse of proper timing to pass the gate. According to the technical report of teletext broadcast submitted in March, 1981, the time from the leading edge of horizontal synchronizing signal to the first bit of CR is (0.154.+-.0.005)H as shown in FIG. 1, which is given by (56.+-.2)Tc when the transmitting time for one bit of data is represented by 1Tc(=175ns). In other words, the location where the teletext signal is superposed moves by 4Tc on the basis of the leading edge of horizontal synchronizing signal. The horizontal synchronizing signal as the standard employs an oscillation output of synchronizing television signal multiplied by the horizontal AFC, in which movement of 5 to 10Tc is suggested in consideration of disturbance of horizontal AFC caused by the equalizing pulse during the vertical blanking period, a shift of the adjusting point of horizontal AFC, and displacement by the temperature characteristic. Furthermore, in consideration of the displacement by the temperature characteristic or adjustment accuracy of a delay circuit which decides the leading edge of the FC gate pulse from the leading edge of horizontal synchronizing signal, the FC gate pulse further moves largely, so that the wrong FC detection pulse may still be generated.
Since the teletext signals are subjected to the band restriction as shown in FIG. 1-(B), the data signals become the pulse train of sine-wave shape. Now, assuming that the waveform in FIG. 1-(B) is compared with the level shown by the one-dot chain line 8 and then is sliced, the sliced output waveform is as shown in FIG. 1-C, so that the binary NRZ signal prior to the superposition on the television signal is reproduced. The teletext decoder reads such sliced data by the proper sampling clock signal, thereby carrying out data reading operation.
An example of conventional circuit which produces the above slice level 8 and slices the superposed teletext signal is shown in FIG. 3, in which an antenna 101 receives the RF signal and composite video signals corresponding to FIG. 1-A are obtained as an output of a video receiving unit 102. The composite video signals are applied to one terminal of a voltage comparator 12 and further to a positive peak detection circuit 13 and a negative peak detection circuit 14, intermediate voltage of outputs of both peak detection circuits 13 and 14 being applied as the slice level to the other terminal of voltage comparator 12, thereby reproducing the superposed signal into the binary digital signal.
This method, however, has the peak detection circuits 13 and 14 so as to detect the peak of noise when the RF input level is small, thereby often reproducing by mistake the superposed signals. Also, when the group delay characteristic of a video demodulator generates the distortion in waveforms, the peak detection is carried out with respect to the distorted waveform, so that a proper sliced level cannot be set.
The CR of signal pattern of 16 bits: 1010 . . . , is used in common to every country, but FC is different. FIG. 1-(C) shows FC adopted in Japan and FIGS. 1-(D), -(E) and -(F) show FCs ruled by the NORTH AMERICAN BROADCAST TELETEXT SPECIFICATION (to be hereinafter abbreviated to NABTS) which will be adopted in U.S.A. In a case of detecting CR and FC signals to obtain the sliced level, the sliced level will vary according to the FC pattern.